The present invention relates to an electric power converter of a type which has both an AC input and an AC output, and includes a DC three-wire circuit consisting of a positive phase, a neutral phase and a negative phase in a portion of the circuit, and which further includes a power rectifier and a power inverter sharing an AC wire and a DC neutral phase.
In an electric power converter for converting an AC input into a DC output or for converting a DC input into an AC output sharing an AC input wire, a DC neutral phase and an AC output wire, as disclosed in Japanese Patent Laid-Open No. 15171/1993, a power rectifier and a power inverter are connected in a half-bridge configuration, so that a current of an AC phase connected to a DC neutral phase flows into a capacitor or flows out of the capacitor in the DC circuit. This electric power converter can be constructed so as to have a small size, as compared to one in which the power rectifier and the power inverter are connected in a full-bridge configuration, and has been put into practical use in the small-capacity electric power converters.
Problems inherent in the prior art will now be described with reference to FIGS. 11 to 18.
FIG. 11 illustrates an electric power converter of a single-phase AC input type in which power is input from a single-phase AC power source connection unit 1 and power is output the power to a load connection unit 32 through a power rectifier, a three-wire DC circuit consisting of a positive phase, a neutral phase and a negative phase, and a power inverter. The power rectifier is constituted by a single-phase AC power source connection unit 1, transistors 7 and 8 connected in series between the positive phase and the negative phase of the DC circuit, diodes 11 and 12 connected in inverse parallel arrangement with the transistors 7 and 8, a reactor 5 connected between the neutral points of the transistors 7, 8 and a first phase of the single-phase AC power source connection unit 1, capacitors 15 and 16 connected in series between the positive phase and the negative phase with their neutral point being connected to the second phase of the single-phase AC power source connection unit 1 and to the neutral phase of the DC circuit, and a capacitor 3 connected in parallel with the single-phase AC power source connection unit 1. That is, a power rectifier working as an electric power converter is constituted as a half-bridge type converter circuit (often called a half-bridge type single-phase power rectifier) to convert a single-phase alternating current into a direct-current three-wire output.
The power inverter is constituted by transistors 17 and 18 connected in series between the positive phase and the negative phase of the DC circuit, diodes 21 and 22 connected in inverse parallel relationship with the transistors 17 and 18, a reactor 25 connected between the neutral point of the transistors 17, 18 and a first phase of the load connection unit 32, two capacitors 15 and 16 connected in series between the positive phase and the negative phase of the DC circuit with their neutral point being connected to the second phase of the single-phase AC power source connection unit 1 and to the neutral phase of the DC circuit, and a capacitor 27 connected in parallel with the load connection unit 32.
To form a noise filter for the load connection unit 32, a capacitor 29 is connected between ground and the first phase of the load connection unit 32, and a capacitor 30 is connected between ground and the second phase of the load connection unit 32.
Upon turning the transistors 7, 8, 17 and 18 on and off, the AC input current ideally assumes a sinusoidal waveform, and the AC output voltage assumes a sinusoidal waveform, too.
In the following description, the single-phase AC power source connection unit 1 is referred to as single-phase AC power source 1, and the load connection unit 32 is referred to as load device 32.
Here, a potential of the first phase of the load device 32 will be described with reference to FIGS. 12 to 14.
FIG. 12 is a vector diagram of a case where the second phase of the single-phase AC power source 1 is grounded, and the output voltage Vuvo of the power inverter is in phase with the input voltage Vuvi of the single-phase AC power source 1. The second phase that is grounded assumes ground potential, and the output voltage Vuvo of the power inverter is in phase with the input voltage Vuvi of the single-phase AC power source 1. Accordingly, the maximum potential of the first phase of the load device 32 becomes equal to the maximum potential of the input voltage of the single-phase AC power source 1.
FIG. 13 is a vector diagram of a case where the second phase of the single-phase AC power source 1 is grounded, and the output voltage Vuvo of the power inverter is in a reverse phase with respect to the input voltage Vuvi of the single-phase AC power source 1. The second phase that is grounded assumes ground potential, and the output voltage Vuvo of the power inverter is in a reverse phase with respect to the input voltage Vuvi of the single-phase AC power source 1. Here, however, the maximum potential of the first phase of the load device 32 is equal to the maximum potential of the input voltage of the single-phase AC power source 1.
FIG. 14 is a vector diagram of a case where the first phase of the single-phase AC power source 1 is grounded, and the output voltage Vuvo of the power inverter is in a reverse phase with respect to the input voltage Vuvi of the single-phase AC power source 1. The first phase that is grounded assumes ground potential, and the output voltage Vuvo of the power inverter is in a reverse phase with respect to the input voltage Vuvi of the single-phase AC power source 1. Therefore, the maximum potential of the first phase of the load device 32 becomes twice as great as the maximum potential of the input voltage of the single-phase AC power source 1.
The potential of the first phase of the load device 32 is applied to the capacitor 29. When the input voltage to the load device 32 is 100 V, the voltage applied to the capacitor 29 is 100 V in FIGS. 12 and 13, but is 200 V in FIG. 14. Thus, an excess voltage is applied to the capacitor 29 in the case of FIG. 14, i.e., the capacitor is often damaged. An arrester may be connected instead of the capacitor. In this case, too, however, the arrestor may often be damaged.
FIG. 15 illustrates an electric power converter of the three-phase AC input type which inputs power from a three-phase AC power source 2 and outputs power to a load device 33 through a power rectifier, a three-wire DC circuit consisting of a positive phase, a neutral phase and a negative phase, and a power inverter. The power rectifier is constituted by the three-phase AC power source connection unit 2, transistors 7 and 8 connected in series between the positive phase and the negative phase of the DC circuit, diodes 11 and 12 connected in inverse parallel relationship with the transistors 7 and 8, a reactor 5 connected between a neutral point of the transistors 7, 8 and the first phase of the three-phase AC power source 2, transistors 9 and 10 connected in series between the positive phase and the negative phase of the DC circuit, diodes 13 and 14 connected in inverse parallel relationship with the transistors 9 and 10, a reactor 6 connected between a neutral point of the transistors 9, 10 and the third phase of the three-phase AC power source 2, two capacitors 15 and 16 connected in series between the positive phase and the negative phase with their neutral point being connected to the second phase of the three-phase AC power source 2 and to the neutral phase of the DC circuit, a capacitor 3 connected in parallel between the first phase and the second phase of the three-phase AC power source 2, and a capacitor 4 connected in parallel between the second phase and the third phase. That is, a power rectifier working as a power converter is constituted having two half-bridge type converter circuits to convert a three-phase alternating current into a DC three-wire output.
The power inverter is constituted by transistors 17 and 18 connected in series between the positive phase and the negative phase of the DC circuit, diodes 21 and 22 connected in inverse parallel with the transistors 17 and 18, a reactor 25 connected between a neutral point of the transistors 17, 18 and the first phase of the load device 33, transistors 19 and 20 connected in series between the positive phase and the negative phase of the DC circuit, diodes 23 and 24 connected in inverse parallel relationship with the transistors 19 and 20, a reactor 26 connected between a neutral point of the transistors 19, 20 and the third phase of the load device 33, two capacitors 15 and 16 connected in series between the positive phase and the negative phase of the DC circuit with their neutral point being connected to the second phase of the three-phase AC power source 2 and to the neutral phase of the DC circuit, a capacitor 27 connected in parallel between the first phase and the second phase of the load connection unit 33, and a capacitor 28 connected in parallel between the second phase and the third phase.
To form a noise filter for the load device 33, a capacitor 29 is connected between ground and the first phase of the load device 33, a capacitor 30 is connected between ground and the second phase of the load device 33, and a capacitor 31 is connected between ground and the third phase of the load device 33.
Here, the potential of the first phase of the load device 33 will be described with reference to FIGS. 16 to 18.
FIG. 16 is a vector diagram of a case where the second phase of the three-phase AC power source 2 is grounded, and the output voltages Vuvo, Vvwo and Vwuo of the power inverter are in phase with the input voltages Vuvi, Vvwi and Vwui of the three-phase AC power source 2. The second phase that is grounded assumes ground potential, and the output voltage of the power inverter is in phase with the input voltage of the three-phase AC power source 2. Therefore, the maximum potential of the first phase of the load device 33 becomes equal to the maximum potential of the input voltage of the three-phase AC power source 2.
FIG. 17 is a vector diagram of a case where the second phase of the three-phase AC power source 2 is grounded, and the output voltages Vuvo, Vvwo and Vwuo of the power inverter are in a reverse phase with respect to the input voltages Vuvi, Vvwi and Vwui of the three-phase AC power source 2. The second phase that is grounded assumes ground potential, and the output voltage of the power inverter is in a reverse phase with respect to the input voltage of the three-phase AC power source 2. Therefore, the maximum potential of the first phase of the load device 33 becomes equal to the maximum potential of the input voltage of the three-phase AC power source 2.
FIG. 18 is a vector diagram of a case where the first phase of the three-phase AC power source 2 is grounded, and the output voltages Vuvo, Vvwo and Vwuo of the power inverter are in a reverse phase with respect to the input voltages Vuvi, Vvwi and Vwui of the three-phase AC power source 2. The first phase that is grounded assumes ground potential, and the output voltage of the power inverter is in reverse phase with the input voltage of the three-phase AC power source 2. Therefore, the maximum potential of the first phase of the load device 33 becomes twice as great as the maximum potential of the input voltage of the three-phase AC power source 2.
The potential of the first phase of the load device 33 is applied to the capacitor 29. When the input voltage to the load device 33 is 200 V, the voltage applied to the capacitor 29 is 200 V in FIGS. 16 and 17, but is 400 V in FIG. 18. Thus, an excess voltage is applied to the capacitor 29 in the case of FIG. 18, i.e., the capacitor is often damaged. An arrester may be connected instead of the capacitor. In this case, too, however, the arrestor may often be damaged.
The half-bridge type power converter sharing an AC wire and a neutral phase of the DC circuit can be realized in a small size, but the potential of the load device varies depending upon which phase of the AC power source is grounded, subjecting the capacitor or the arrester connected between the load device and ground to damage, as described above. So far, this problem has been coped with by confirming the grounded phase at the time of installing the power converter, however, this involves an inconvenience in that the grounded phase must be confirmed every time the input power source is renewed. Therefore, proper ground detection and protection have been desired from the standpoint of maintaining the reliability of the system.